The distribution of positron emitting substances in brain can be followed by positron emission tomography (PET). We are developing 18F-labeled high affinity opiate drugs to be injected into living humans for the visualization of opiate receptor patterns in vivo. It will be interesting to determine whether opiate receptor distribution patterns in cortex change as a function of attention and emotional states. Meanwhile, we have carefully documented the receptor-binding properties and kinetics of 3H-cyclofoxy in rat brain after in vivo injection.